Ammonia Losses from Surface-Applied Nitrogen Fertilizer as Controlled by Soluble Calcium and Magnesium: General Theory1

L. B. Fenn,

R. M. Taylor and

J. E. Matocha2

Abstract

Abstract

Ammonia volatilization from soils after surface application of urea or inorganic N was reduced by Ca and Mg nitrates or chlorides. Sulfates were only slightly effective. Suppression of NH3 loss was a result of two chemical reactions: (i) precipitation of CO3 by Ca, thus preventing permanent (NH4)2CO3·H2O (AC) formation, and (ii) Ca depression of soil pH by depression of the dissociation of the CaCO3-Ca(OH)2 buffer system. Soil pH values as low as 6.5 were recorded in calcareous soil and 3.9 and 4.5 in the acid soils after addition of Ca. Soil pH values in the presence of Ca reached approximate neutrality in 24 hours even in acid soils. Soil pH without Ca but with AC increased to > 9.0 in every soil. Addition of AC plus Ca reduced the pH of calcareous soil to 5.3 and raised it to 5.7 in acid soils. Calcium carbonate precipitation eventually occurred in acid soils, preventing high NH3 losses. Evidently, CaCO3 precipitated on the surface of acid soils after the pH reached a certain critical value and, as expected, the pH was maintained near that of the pure CaCO3 system.

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